Electromagnetic flow meters, and their methods of operation, are well known. In use, an electromagnetic field is generated whose lines of flux are mutually perpendicular to the longitudinal axis of the flow conduit through which the fluid to be metered is conducted and to the transverse axis along which the electrodes are normally located at opposite positions with respect to the conduit. The operating principles are based on Faraday's law of induction, which states that the voltage induced across any conductor as it moves at right angles through a magnetic field will be proportional to the velocity of that conductor. The metered fluid effectively constitutes a series of fluid conductors moving through the magnetic field; the more rapid the rate of flow, the greater the instantaneous values of the voltage established at the electrodes.
Electromagnetic flow metering is a mature and robust technology with the first commercial units being sold over 50 years ago. However, the cost of electromagnetic flow meters is relatively high due to the materials used in their construction and necessarily high labor costs.
Existing commercial electromagnetic flow meters typically employ two coils positioned on either side of the flow conduit in order to generate an homogenous magnetic field across the flow conduit. The coils may be mounted on the outside of the flow conduit or on the inside of the flow conduit.
When the coils are mounted on the outside of the flow conduit, the flow conduit has to be made from a material through which the magnetic field generated by the coils can pass and it must be able to withstand the operating pressure of the fluid flowing inside the conduit. Stainless steel tubing is commonly used because of its strength and non-magnetic properties. An insulating liner within the flow conduit is also provided to ensure that the electric field generated by the moving conducting liquid does not short to ground.
When the coils are mounted on the inside of the flow conduit an insert is usually provided which defines a measuring section having a narrower effective diameter than the diameter of the flow conduit. This reduced diameter measuring section provides a space between the insert and the flow conduit in which the coils can be mounted. The insert is typically manufactured by molding or extrusion using a plastics or rubber material. A stainless steel tube may be provided around the measuring section of the insert to provide the required pressure containment and/or the space between the insert and the flow conduit may be back-potted with a suitable potting compound.
Whilst smaller flow meters (those having a diameter of less than about 350 mm) can employ both techniques, larger flow meters (those having a diameter greater than about 350 mm) tend to employ coils mounted on the outside of the flow conduit due to the ease of manufacture.